This application relates to flexible magnetic disc recorders, and particularly to flexible recording discs housed in a protective cassette, such as generally disclosed in U.S. Pat. Nos. 3,833,926 and 3,947,893. Such discs have been used, for example, to record and play back individual frames of video information (two interlaced fields). In accordance with U.S. standards a single field can be stored on a circular track with the disc rotating at 30 revolutions per second (1800 rpm). The speed varies somewhat with standards in other countries, but the principles are basically the same.
For various reasons it may be desirable to rotate the recording disc much faster. One case is in a single field recording, where the rpm of the disc must be doubled, to 3600 rpm in the U.S. Even the 30 rpm speeds are considerably faster than those normally encountered in digital recording, such as used in data processing. As the rotational speed of the thin flexible discs increases the dynamics of the disc become more important. At speeds in the order of 3000 rpm and above considerable centrifugal force is exerted on the hub and the central area of the disc. Also, when the disc is rotated within a confined region, as in a cassette there is much more significant tendancy to pump air outward along the disc surfaces. Any unbalance in air pressure on either side of the disc can result in forces which tend to deflect the disc from the plane which it seeks due to centrifugal force.
Coupled with these factors is the desirability of having recording disc units which are interchangeable in the recorder. The vertical interval (retrace) information indicates the beginning and/or end of each field of video information. It is desirable to have such information generally aligned track for track, e.g., radially of the disc. This in turn requires that the disc always achieve the same relative position to the drive hub, from which the discs are to be removed and interchanged.